The present invention relates generally to piezoelectric transducers such as audible alarms or indicators and, more particularly, to a self-calibrating system for determining the optimum driving frequency (resonant frequency) of an individual transducer and for operating the transducer at that frequency.
A number of different forms of audible indicator or alarm units employing piezoelectric elements or transducers to generate a relatively piercing and noticeable audible tone when energized with relatively little power have come into use. Such indicators are commonly used in numerous small and large electrical appliances, alarm systems of various types, and for other applications in which the generation of an audible signal is required. By way of example, the following U.S. patents are identified for their disclosures of such audible indicator units: Mallory U.S. Pat. No. 3,569,963; and Salem U.S. Pat. No. 4,164,735.
Desirably, such systems are operated at or near the mechanical resonant frequency of the vibrating piezoelectric element. While the units may be operated at other frequencies, the most efficient use of available electrical energy and greatest power output results from operation at or near the resonant frequency.
However, individual piezoelectric transducer units, even of the same manufacturer's type number, often vary in precise operating frequency. Typically, the manufacturer's data specifies only a frequency range within which the actual resonant or optimum frequency is guaranteed to lie. Further, the resonant frequency of a single unit itself may vary due to such factors as aging, and varying temperature and humidity. In view of this, various systems have been proposed to operate the alarm units at the mechanical resonant frequency, either through manual or automatic adjustment. Automatic adjustment systems are particularly advantageous, although generally more costly. Although not audible indicators, the following two U.S. patents are identified for their disclosures of systems for automatically driving a piezoelectric transducer or element at its optimum frequency: Kawada U.S. Pat. No. 3,743,868 and Watanabe et al U.S. Pat. No. 3,967,143.
As noted above, audible indicator or alarm circuits are commonly used in a number of different products, including large and small appliances. In recent years, microprocessor-based control systems have been proposed for and even implemented in these various products, as an alternative to previously employed mechanical timers, program sequencers, and the like. Typical microcomputers used in such applications are relatively small and inexpensive, and operate under control of a program permanently stored in read-only memory (ROM) at the time of manufacture following careful design evaluation. Once manufactured with a particular program, these devices are essentially dedicated-function digital logic devices. It will be appreciated that, through design, such systems are capable of responding in potentially an unlimited number of ways to external inputs to potentially provide an unlimited number of control functions.
It is a characteristic of such systems that hardware costs can remain essentially constant even though various different functions and features may be provided, so long as the software programming instructions needed to accomplish the desired functions fit within available program memory space, without requiring an incremental increase in hardware cost such as occurs when a limit to memory capacity is reached, and another block of memory is required to implement a particular additional function.